1. Field of the Invention
The present invention relates to optical add-drop multiplexers, and particularly to an optical add-drop multiplexer incorporating a paraboloid reflector.
2. Description of Related Art
A n optical add-drop multiplexer (OADM) is used in a node of a wavelength division multiplexing (WDM) communications network. The OADM extracts a channel containing information conveyed to the node from a multiplexed stream, and allows remaining channels to pass through unaltered. The OADM is also needed to add a new channel, which transports information generated within the node to the multiplexed stream. In practice, the dropped channel and the added channel are allocated the same wavelength, to simplify network management. A user can readily associate the dropped and added channels with the concerned node. An other application of OADMs is in routing nodes of reconfigurable optical networks. Certain information streams can be re-routed according to changed traffic conditions, or to remedy a failure downstream of the connected node.
The OADM can be considered as a four-port device, with two ports for the input and output of the overall stream and two ports for the input and output of an individual channel. The ports related to the overall stream are hereinafter called the input port and the output port, and the ports related to the individual channel are hereinafter called the drop port and the add port.
Nowadays, OADMs which can add and drop a desired channel in a multiplexed stream containing several wavelength channels are becoming more and more popular for use in WDM networks. Presently, non-absorbing interference filters are used to manufacture such OADMs. These filters comprise a series of dielectric layers. The layers have refractive indices and thicknesses such that the filters transmit a certain portion of the spectrum of incident rays, and reflect the remaining portion. A simple inclined interference filter placed in a suitable system for collimating and re-focusing the optical beams can directly perform the functions required by a WDM device. A first face of the filter receives the multiplexed stream. The first face reflects all wavelengths except the one to be extracted, which exits an opposite second face of the filter. The first face also receives the wavelength to be inserted, which is then transmitted through the filter and is combined with the multiplexed stream. However, a WDM device using a single filter has a poor reflection extinction ratio. A fraction of the signal of the extracted channel is transferred to the output port, and interferes with the signal having the same wavelength that is added at the add port. This interference can seriously degrade the performance of the system. Moreover, tuning of the filter is difficult. Variation of the inclination of the filter is required to vary the transmitted bandwidth, but such variation also causes angular displacement of the reflected beam relative to an optical collecting device of the system.
U.S. Pat. No. 5,812,291 discloses an optical tunable add-drop multiplexer comprising a pair of identical interference filters. One of the filters carries out the drop function, and the other filter carries out the add function. The two filters are arranged in parallel planes, and are secured to opposite faces of a transparent body. Therefore, the stream comprising a plurality of channels undergoes reflection by both filters when it travels through the device. The transparent body is mounted on a support which is hinged on an axis. The axis is parallel to the planes of the filters and passes through the barycenter of the multiplexer. The body is rotatable in either direction to vary the tuning wavelength of the filters. Unfortunately, the structure of the multiplexer is complicated. In addition, wavelength selection is troublesome and inaccurate, due to the method of mechanical tuning.